In Japanese anime and pop culture, "X" generally means a collaboration, cross-over or conjoining of two factions, ideas, or characters.
More specifically, in some anime and manga titles it is referred to as a "cross", E.G., "Hayate X Blade" as "hunter cross blade".
It can also mean "and", such as in a pairing of two characters. E.G., the anime "Romeo X Juliet".
It could mean nothing, as in the wildly successful "Hunter X Hunter" manga that is simply pronounced as "hunter hunter". Although, some believe it means "versus" in this case.
Still, it could simply be a way to drum up discussion. For example, in the manga "High School DxD", "DxD" is pronounced "Di Di". Initially, it was believed "DxD" stood for "Dragon of Dragon" (ドラゴン・オブ・ドラゴン, Doragon obu Doragon). Later on it was revealed that "DxD" refers to an organization that was comprised of two factions. While others simply refuse to believe any of that and insist it stands for "DD", as in double-D bra size, referring to the copious amount of fanservice in the series.
Either way, I like the sound of "Mondo X SteelBook".